1. Field of the Invention
The present invention relates to characterizing the latency of an audio channel and, more particularly, to actively characterizing the latency of an audio channel.
2. Background Information
A copy of a signal sample stream that has been applied (after digital-to-analog conversion) to a loudspeaker is sometimes employed as a reference channel for various processing techniques in situations where a microphone receives the audio output signal of the loudspeaker. For example, in a communications system, if locally the microphone and loudspeaker operate at the same time and the far end of the communications system operates in a similar manner, a positive feedback loop may result without additional processing. It is commonly desired that, for such additional processing techniques, the reference channel signal sample stream be correlated in time with the signal sample stream produced by the microphone on a sample-by-sample basis. Processing techniques, such as acoustic echo cancellation (AEC), for example, fall into this category. For these processing techniques, ultimately, the signal samples produced by the microphone (after analog-to-digital conversion) should be paired with signal samples in the reference channel corresponding to the audio output signal being produced by the loudspeaker.
Personal computers are more frequently being employed in communications applications. For example, a personal computer may be employed in audio conferencing because state-of-the-art personal computers frequently come equipped with a microphone, loudspeakers, or alternatively, these devices may be added to a personal computer relatively easily. For example, the availability of computer add-in cards, such as a soundcard or a card that permits the personal computer to successfully interface with an Integrated Services Digital Network (ISDN) line or Plain Old Telephone Service (POTS) line, allows the personal computer to be employed in this fashion. For personal computers employed in this way, it is desirable to enable full duplex audio operation. Where acoustic coupling exists between a microphone and a speaker that are also coupled to a communications medium, acoustic echo cancellation techniques are typically employed.
To accomplish acoustic echo cancellation on a personal computer, for example, the previously described time correlation is performed. Unfortunately, techniques to perform this time correlation for a personal computer usually involve using detailed knowledge regarding aspects of the personal computer hardware and software employed. A reason this occurs is because specific aspects of the hardware and/or software may affect signal latency. For example, a signal or stream of signal samples produced in an audio channel before or ahead of a speaker in the channel may later be reproduced in the audio channel after a microphone. Therefore, the hardware and/or software may play a part in performing the previously described time correlation. For example, in one approach, the signal samples for the input stream and the output stream are paired in the low level software that controls the physical layer of the digital-to-analog converter. See U.S. Pat. No. 5,495,473, titled “Methods and Apparatus For Implementing Hardware Independent Echo Cancellation In A Full-Duplex Audio Stream in a Computer System”, by Daniel R. Cox, issued Feb. 27, 1996, assigned to the assignee of the current invention and herein incorporated by reference. Other hardware or software specific approaches may, likewise, be employed.
To avoid this low-level correlation control, a technique or approach is needed to provide this time correlation independent of the underlying hardware and software for a computer, such as a personal computer.